Semiconductor light emitting device and manufacturing method therefor

ABSTRACT

A semiconductor light emitting device of double hetero junction comprising an active layer; and clad layers comprising an n-type layer and p-type layer, the clad layers sandwiching the active layer, a band gap energy of the clad layers being larger than that of the active layer; wherein band gap energy of the n-type clad layer is smaller than of the p-type clad layer.

BACKGROUND OF THE INVENTION

The invention relates to a semiconductor laser having a doubleheterodyne structure. More particularly, the invention relates to asemiconductor laser which uses a semiconductor of gallium nitride typecompound suitable for emission of blue light, which is capable ofreducing operating voltage without reducing the light emittingefficiency.

In the past, blue LED had a fault in putting it to practical use becauseit has lower luminance than a red LED or a green LED, but in recentyears the luminance of the blue LED has increased and is in thespotlight now as a semiconductor of gallium nitride type compound hasbeen in use, making it possible to obtain in p-type semiconductor layerof a low resistance containing Mg as a dopant.

The semiconductor of gallium nitride type compound described here isreferred to a semiconductor in which a compound of Ga of group IIIelement and N of group V element or part of Ga of group III element issubstituted by other group III element such as Al and In and/or asemiconductor in which part of N of group V element is substituted byother group V element such as P and As.

In a conventional manufacturing method, gallium nitride type LEDs weremanufactured in such processes as described below, and a perspectiveview of LED which uses a semiconductor of completed gallium nitride typecompound is shown in FIG. 11.

First, by the organometallic compound vapor phase growth method(hereinafter referred to as MOCVD method), the carrier gas H₂ togetherwith trimethyl gallium which is an organometallic compound gas(hereinafter referred to as TMG), ammonia (NH₃) and SiH₄ and the likeare supplied as a dopant to a substrate consisting, for example, ofsapphire (single crystal Al₂O₃) at low temperature of 400° to 700° C.,approximately 0.10 to 0.2 μm of low temperature buffer layer 2consisting of n-type GaN layer is formed, and then the same gas issupplied at high temperature of 700° to 1200° C., and approximately 2 to5 μm of high temperature buffer layer 3 consisting of n-type GaN of thesame composition is formed. The low temperature buffer layer 2 is formedby polycristalline layer to ease the strain caused by mismatching of thelattice between a substrate 1 and the single crystal layer ofsemiconductor and then turned into a single crystal by being subjectedto 700° to 1200° C., in order to match the lattice by laminating thesingle crystal of the high temperature buffer layer 3 on that singlecrystal.

Further, material gas of trimethyl aluminium (hereinafter referred to asTMA) is added to the foregoing gas, a film of an n-type Al_(k)Ga_(1-k)N(0<k<1) layer containing S of n-type dopant is formed, so thatapproximately 0.1 to 0.3 μm of an n-type clad layer 4 is formed to forma double heterodyne junction.

Then, instead of TMA which is the foregoing material gas, trimethylindium (hereinafter referred to as TMI) is introduced to formapproximately 0.05 to 0.1 μm of an active layer 5 consisting, forexample, of In_(y)Ga_(1-y)N (0<y<1), a material whose band gap issmaller than that of the clad layer.

Further, impurity material gas is substituted by SiH₄ using the samematerial gas used for forming the n-type clad layer 4, Mg as a p-typeimpurity of biscyclopentadiene magnesium (hereinafter referred to asCp₂Mg) or dimethyl zinc (hereinafter referred to as DMZn) for An isadded and introduced into a reaction tube, causing a p-typeAl_(k)Ga_(1-k)N layer which is a p-type clad layer 6 to be grown invapor phase. By this process, a double hetero junction is formed by then-type clad layer 4, active layer 5, and p-type clad layer 6.

Next, in order to form a contact layer (cap layer) 7, Cp₂Mg or DMZn issupplied as the impurity material gas using the same gas as theforegoing buffer layer 23 to form 0.3 to 2 μm of the p-type GaN layer.

Afterward, a protective film such as SiO₂ and Si₃N₄ is provided all overthe surface of the grown layer of a semiconductor layer, aniline orelectron is irradiated at 400° to 800° C. for approximately 20 to 60minutes to activate the p-type clad layer 6 and the contact layer (caplayer) 7, after the protective film is removed, resist is applied andpatterning is provided to form an electrode on the n-side, part orrespective grown semiconductor layers is removed by dry etching so as toexpose the buffer layer 3 or the n-type clad layer 4 which is the n-typeGaN layer, an electrode 8 on the n-side and an electrode 9 on the p-sideare formed by sputtering and the like, and AN LED chip is formed bydicing.

As a conventional semiconductor laser, one that uses a semiconductor ofGaAs type compound is known, in which a resonator is formed by a doublehetero junction structure with both sides of an active layer being heldbetween clad layers consisting of a material having greater band gapenergy and, smaller refractive index than the material of such activelayer, so that it is possible to obtain the light oscillated in suchresonator. Shown in FIG. 12 is an example of a semiconductor laser whichuses a semiconductor of GaAs type compound having a refractive indexwave guide structure provided with a difference of refractive index byan absorption layer in order to confine the light in the stripe portionof the active layer.

In FIG. 12, the numeral 121 represents a semiconductor substrateconsisting, for example, of an n-type GaAs, on which are laminated inorder a lower clad layer 124 consisting, for example, of an n-typeAl_(α)Ga_(1-α)As (0.35≦α≦0.75), an active layer 125 consisting, forexample, of Al_(β)Ga_(1-β)As (0<β≦0.3) of non-doping type or an n-typeor a p-type, a first upper clad layer 126 a consisting of a p-typeAl_(α)Ga_(1-α)As, a current laminating 120 consisting of an n-type GaAs,a second upper clad layer 126 b consisting of a p-type Al_(α)Ga_(1-α)As,and a contact layer (cap layer) 127 consisting of a p-type GaAs, and thep-side electrode 128 and the n-side electrode 129 are respectivelyprovided on the upper surface and the lower surface in order to form achip of a semiconductor laser. In this structure, the current limitinglayer 120 consisting of the n-type GaAs restricts the injection currentto the stripe-like active area of width W, by absorbing the lightgenerated by the active layer, a difference of refractive index isprovided in the inside and the outside of the stripe. Therefore, thesemiconductor laser of the present invention is used as a semiconductorlaser of a red or infrared ray refractive index wave guide structurewherein the light is confined in transverse direction and the wave ofstripe-like active area of width W is directed stably.

In the semiconductor laser of such structure, a blue light radiatingsemiconductor laser using a semiconductor of gallium nitride typecompound is also requested.

In a conventional semiconductor of gallium nitride type compound, thelight emitting efficiency of the light emitting element of double heterojunction is high but the operating voltage thereof is high. If amaterial of small band gap energy, that is, a material of small Alcomposition rate k of Al_(k)Ga_(1-k)N is used for the n-type clad layerand the p-type clad layer in order to lower the operating voltage, theoperating voltage is lowered but the electron outflow from the activelayer to the p-type clad layer increases, while the light emittingefficiency is lowered.

In the case where a semiconductor laser is to be composed by using asemiconductor of gallium nitride type compound, by providing a structurewherein an active layer is interposed between both sides by a clad layerconsisting of a material having greater band gap energy and smallerrefractive index than such active layer so as to confine the light inthe active layer for oscillation, it can be considered to useIn_(y)Ga_(1-y)N (1<y<1,where y=0.1 for example) as the active layer andAl_(k)Ga_(1-k)N (0<k<1, where k=0.2 for example) as the clad layer ofboth sides.

In a semiconductor laser which uses a conventional semiconductor ofarsenic gallium type compound, specific resistance of Al_(α)Ga_(1-α)Asas the clad layer is approximately 100 Ω·cm and there occurs no problemof increased operating voltage or heat generation even if such cladlayer is used as one requiring approximately 1 to 2 μm, but if asemiconductor of gallium nitride type compound is used, the specificresistance of Al_(k)Ga_(1-k)N (k=0.2 for example) is approximately 1000Ω·cm when the carrier density of 10¹⁷ cm⁻³, which is approximately 8times as compared with the specific resistance of GaN of the samecarrier density, thereby increasing the operating voltage as well as thepower consumption in addition to the problem of heat generation.

Further, in a light emitting element of a semiconductor which uses aconventional semiconductor of gallium nitride type compound wherein theGaN layer is used as the contact layer 7 in which the p-side electrodeis to be made, due to such reasons that the GaN layer is affected byvariations of the surface level and that there is a large energy gapbetween the metallic conduction band such as the alloy of Au or Au andZn used as electrode and the valence band of GaN, and the contactresistance between the electrode metal and the cap layer does notstabilize as a result, so that the contact resistance becomes large andthe operating voltage also rises. These problems results from a basicproblem that it is not possible to increase the carrier density of thep-type layer, and further, in a type of semiconductor laser with thecurrent injection area being restricted to stripe-like shape in whichthe contact area of the electrode is formed into a stripe-like shape,and the problem becomes more conspicuous.

Furthermore, as described above, the light emitting element of asemiconductor which uses a conventional semiconductor of gallium nitridetype compound is composed by laminating on a sapphire substrate asemiconductor layer of gallium nitride type compound by means of a lowtemperature buffer layer consisting of GaN and a high temperature bufferlayer, the lattice constant 4.758 Å of the sapphire substrate is largelydifferent from the lattice constant 3.189 Å of GaN, the interatomicbonding strength of GaN is strong although it is weaker than that ofAlGaN group, so that a crystal defect or transition is likely to occurdue to temperature shock. In case a crystal defect or transistion occursin the low temperature buffer layer, the crystal defect or transitionprogresses to the semiconductor layer formed thereon, therebydeteriorating the light emitting characteristic and reducing the life.

In addition, in the light emitting element of a semiconductor which usesa conventional semiconductor of gallium nitride type compound, asdescribed above, electric current flows between the p-side electrode 8provided on the contact layer 7 and the n-side electrode 9 provided onthe high temperature buffer layer 3 which is an n-type layer due to thevoltage applied therebetween, and the electric current flowing to then-side electrode 9 has high carrier density of the buffer layers 2 and3, so that the electric current flows throughout the buffer layers 3 and2. On the other hand, because the buffer layer, the low temperaturebuffer layer 2 in particular is formed on a substrate consisting, forexample, of sapphire that has a different lattice constant from that ofa semiconductor of gallium nitride type compound, crystal defects ortransition are likely to occur. When electric current flows into thebuffer layer where crystal defects or transition take place, crystaldefects or transition increase further due to the heat generated by theelectric current, such crystal defects or transition progress to thesemiconductor which contributes to the emission of light, therebylowering the light emitting characteristic, reliability or the life.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a light emitting device(hereinafter referred to as a light emitting element) of a semiconductorusing a highly characteristic semiconductor of gallium nitride typecompound which prevents increase in series resistance, that is, anincrease in operating voltage resulting from the use of a semiconductorof gallium nitride type compound as described above and restrictsfurther the occurrence of crystal defects or transition.

A first object of the present invention is to provide a light emittingelement of a semiconductor of double hetero structure in which the lightemitting efficiency does not lower with the operating voltage being low.

A second object of the present invention is to provide a semiconductorlaser of high light emitting efficiency which demonstrates sufficientlythe effect of confining the light in an active layer by a clad layereven in a semiconductor laser which uses a semiconductor of galliumnitride type compound and is suitable for emission of blue light, lowersthe series resistance of the semiconductor layer, and operates on lowoperating voltage.

A third object of the present invention is to p provide a light emittingelement of a semiconductor in which the contact resistance between then-side electrode and the contact layer is small and a large output canbe obtained from low operating voltage.

A fourth object of the present invention is to provide a light emittingelement of a semiconductor of high characteristic and long life whichrestricts occurrence of crystal defects or transition by reducingfurther the strain of the buffer layer on the surface of a substrateconsisting of sapphire and the like and prevents the progress of crystaldefects or transition toward the semiconductor layer which contributesto emission of light.

The light emitting element of a semiconductor which realizes the firstobject of the present invention has at least a sandwich structureconsisting of an n-type clad layer, an active layer, and a p-type cladlayer, is light emitting element of a semiconductor of double heterojunction type which is formed by a material, band gap energy of saidactive layer being smaller than the band gap energy of both theforegoing clad layers, and both the foregoing clad layers being selectedso that the band gap energy of the foregoing n-type clad layer becomessmaller than the band gap energy of the foregoing p-type clad layer.

It is preferable that the foregoing n-type clad layer consists of ann-type Al_(x)Ga_(1-x)N (0≦x≦0.5), that the foregoing active layerconsists of In_(y)Ga_(1-y)N (0≦y≦1), that the foregoing n-type cladlayer consists of a p-type Al_(z)Ga_(1-z)N (0<z≦1), where 2x≦z.

It is preferable that a buffer layer consisting of GaN is providedbetween one of the foregoing clad layers and the substrate in order thatit is possible to relieve the strain of the clad layer, preventoccurrence of crystal defects or transition in the clad layer, and lowerthe resistance of the semiconductor layer.

In accordance with the light emitting element of a semiconductor whichrealizes the first object of the present invention, because a materialhaving smaller band gap energy than that of the p-type clad layer isused for the n-type clad layer, injection of electron into the activelayer from the n-type clad layer is carried out easily at low voltage.On the other hand, because a material having a large band gap energy isused for the p-type clad layer in the same way as in the past, escape ofelectron from the active layer to the p-type clad layer is less,contributing to recombination of electron and positive hole in theactive layer. Because the positive hole has a greater effective massthan the electron, there is less escape of the electron toward then-type clad layer side of the positive hole injected into the activelayer even if the band gap energy of the n-type clad layer is small.Therefore, the electron contributes to recombination in the active layerwithout wasting the positive hole, so that it is possible to lower theoperating voltage by the amount of the band gap energy of the n-typeclad layer is reduced, and the light of approximately the same luminanceis emitted.

Because the rate of the band gap energy of the n-type clad layer can be,made small is approximately three times the electron having effectivemass of the positive hole, it is possible to reduce the band gap energyof the n-type clad layer to approximately half the difference of theband gap energy between the p-type clad layer and the active layer, ifthe Al_(k)Ga_(1-k)N material is used, it is possible to reduce the ratiok of Al to less than half the ratio of Al of the p-type clad layer, andit is possible to lower the operating voltage by 5 to 10%.

A light emitting element which realizes the second object of the presentinvention is a semiconductor laser of double hetero junction structurewhich has an active layer, an n-type layer and p-type layer consistingof a material having greater band gap energy and smaller refractiveindex that the active layer, and the foregoing active layer being heldbetween the foregoing n-type layer and the p-type layer, wherein theforegoing n-type layer and the p-type layer respectively consist of atleast two layers, a low refractive index layer consisting of materialwith small refractive index is provided on the foregoing active layerside of the n-type layer and p-type layer, and a low resistance layerhaving smaller electric resistance than the foregoing low refractiveindex layer is provided in other portion of an electric current path ofthe n-type layer and p-type layer.

It is preferable that the thickness of the foregoing low refractiveindex layer is 10 to 50% with respect to the thickness of the foregoingn-type layer or p-type layer, so that it is possible to confine thelight effectively in the active layer to certain extent and to lower theoperating voltage.

Similar to the foregoing, it is preferable that the thickness of theforegoing refractive index layer is 0.1 to 0.3 μm because it is possibleto confine the light effectively in the active layer to certain extentand to lower the operating voltage.

In order to obtain a semicondcutor laser which emits blue light ofexcellent light emitting characteristic, it is preferable that theforegoing active layer consists of Al_(m)Ga_(n)In_(1-m-n)N (0≦m<1,0<n<1, 0<m+n<1), the foregoing low refractive index layer consists ofAl_(r)Ga_(s)In_(1-r-s)N (0≦r<1, 0<s<1,m+n<r+s≦1, m<r), and the foregoinglow resistance layer consists of A_(t)Ga_(n)In_(1-t-u)N (0≦t<1, 0<u≦1,0<t+u<1,m<t<r, m+n<t+u≦r+s).

It is preferable that m=0 in the material composition of the foregoingactive layer, that r+s=1 in the material composition of the foregoinglow refractive index layer, and that t=o and u=1 in the materialcomposition of the foregoing low resistance layer because it is possibleto obtain a semiconductor laser which is of more simple structure, hasexcellent light emitting characteristic, and emits blue light.

In accordance with the semiconductor laser which realizes the secondobject of the present invention, since the band gap energy forsandwiching the active layer is large, a low refractive index layer isprovided on the respective active layer side of the n-type layer andp-type layer consisting of a material with small refractive index, and alow resistance layer of small electric resistance is provided in otherportion which is to be an electric current path, effect for confiningthe light into the active layer is achieved by reflecting the lightefficiently on the low refractive index layer, with respect to anincrease in specific resistance due to the low refractive index layer,it is possible to reduce sufficiently the operating voltage by other lowresistance layer without being influenced excessively by forming the lowrefractive index layer on a thin layer of approximately 0.1 to 0.3 μmfor example.

As a result, it is possible to prevent heat generation due to excessiveresistance loss, improve the light emitting efficiency, and extend thelife.

The light emitting element of a semiconductor which realizes the thirdobject of the present invention is a light emitting element of asemiconductor in which a semiconductor layer of gallium nitride typecompound having a light emitting portion containing at least an n-typelayer and p-type layer is laminated on a substrate and an n-sideelectrode an p-side electrode to be connected respectively to theforegoing n-type layer and p-type layer are provided, and on at leastthe electrode side surface of the semiconductor layer to be providedwith the foregoing p-side electrode are provided a p-typeIn_(a)Ga_(1-a)N (0<a<1) or p-type GaSa or p-type GaP or p-typeIn_(b)Ga_(1-b)As (0<b<1) or In_(b)Ga_(1-b)P (0<b<1).

That the composition ratio of In of the foregoing In_(a)Ga_(1-a)N is0<a≦0.5 is preferable, so that it is possible to lower the contactresistance without emerging the problem of lattice mismatching.

In accordance with the light emitting element of a semiconductor whichrealizes the third object of the present invention, becauseIn_(a)Ga_(1-a)N or GaAs or GaP or In_(b)Ga_(1-b)As or In_(b)Ga_(1-b)P isused on the surface of the contact layer to be provided with the p-sideelectrode, the contact resistance of the semiconductor layer and theelectrode becomes small. In other words, because these semiconductorlayer such as In_(a)Ga_(1-a)N has smaller band gap energy (forbiddenband width) that GaN and is difficult to be oxidized, it becomesdifficult for the surface level to occur and the difficulty of flowingelectric current (contact resistance) due to the trapping of electron orpositive hole according to the surface level is reduced. Further, asemiconductor layer such as In_(a)Ga_(1-a)N has smaller band gap energy(forbidden band width) as compared to GaN and the energy gap between theenergy level of the metallic conduction band as an electrode and thevalence band of the semiconductor layer is small, permitting thepositive hole to flow easily. There is a gap occurring in the energylevel of the valence band between In_(a)Ga_(1-a)N layer and GaN layerbut the energy gap E_(v) between the electrode metal and GaN layer isdivided into the energy gap E_(v1) between the metal and In_(a)Ga_(1-a)Nlayer and the energy gap E_(v2) between In_(a)Ga_(1-a)N layer and GaNlayer, so that the apparent contact resistance becomes small since thepositive hole or electron which has climbed over the small energy gapE_(v1) should climb over the small energy gap E_(v2) further whole itshould not climb over the large energy gap E_(v) directly.

With respect to the GaAs pr GaP, because the band gap energy (forbiddenband width) is small and difficult to be odoxidized in the same manneras In_(a)Ga_(1-a)N, the surface level is difficult to occur and the bandgap energy is smaller than that of In_(a)Ga_(1-a)N, so that the contactresistance becomes small further.

As in the case of the foregoing In_(a)Ga_(1-a)N, In of In_(b)Ga_(1-b)Asor In_(b)Ga_(1-b)P plays a role of reducing the band gap energy(forbidden band width) further and acts to reduce the contact resistancefurther. In this case, lattice matching deviates largely from that ofIn_(a)Ga_(1-a)N but the effect of reduction of the band gap energy islarger. In addition, it is possible to increase the p-type carrierdensity further.

The light emitting element of a semiconductor which realizes the fourthobject of the present invention is a light emitting element of asemiconductor in which a semiconductor layer of gallium nitride typecompound having a luminous portion containing at least an n-type layerand a p-type layer is laminated on a substrate by means of a bufferlayer and at least the foregoing substrate side of the foregoing bufferlayer is consisted of a semiconductor layer of gallium nitride typecompound containing at least one kind of element selected from a groupconsisting of P and As.

It is preferable that the foregoing buffer layer has a low temperaturebuffer layer formed at least at low temperature and that the lowtemperature buffer layer is a semiconductor layer consisting ofIn_(c)Ga_(1-c)N (0<c<1) or In_(d)Al_(e)Ga_(n)In_(1-d-e)N (0<d<1, 0<e<1,0<d+e<1).

It is preferable that the foregoing buffer layer has a low temperaturebuffer layer formed at least at low temperature and that the lowtemperature buffer layer is a semiconductor layer consisting ofGaN_(u)P_(1-u) (0<u<1) or GaN_(v)As_(1-v) (0<v<1) because it is possibleto reduce the straining of the buffer layer.

In accordance with this light emitting element of a semiconductor,because In, P or As is contained in the buffer layer consisting ofsemiconductor of gallium nitride type compound on a sapphire substrate,the buffer layer becomes soft and occurrence of crystal defects ortransition becomes difficult. In other words, when a part of Ga of GaNbecomes In_(c)Ga_(1-c)N (0<c<1) which is substituted by In, In isheavier than Ga and is easily cut in a crystal, the strain is easilyrelieved, making it difficult for crystal defects and the like to occur.In addition, it becomes easy to form a polycrystalline film at lowtemperature by containing In, and it is possible to relieve the strainfurther by forming a buffer layer at low temperature. These phenomenaapply in the same manner to In_(d)Al_(e)Ga_(1-d-e)N (0<d<1, 0<c<1,0<d+c<1) of which part of Ga is substituted by Al.

Further, when part of N of GaN becomes GaN_(u)P_(1-u) (0<u<1) orGaN_(v)As_(1-v) (0<v<1) of which part of N of GaN is substituted by P orAs, P or As becomes heavier than N and easily moves in a crystal, sothat the combination is easily cut. Therefore, according to the samereason that part of the foregoing Ga is substituted by In, the strain ofthe buffer layer is relieved, so that it becomes difficult for crystaldefects or transition to occur.

Crystal defects or transition occurred in a buffer layer where thestrain is likely to occur most in contact with sapphire substrate andthe like progress toward a semiconductor layer which contributes to thelight emitting portion, and the strain of the buffer layer is relievedand the occurrence of crystal defects or transition of the buffer layeris restricted, so that the occurrence of crystal defects or transitionof a semiconductor layer which contributes to the light emitting portionis restricted, the light emitting characteristic is improved, and thelife and also improved.

Other light emitting element of a semiconductor which achieves the forthobject of the present invention consists of a semiconductor layer wherethe electric current is difficult to flow at least on the foregoingsubstrate side of the buffer layer.

That the foregoing buffer layer is a semiconductor layer of galliumnitride type compound containing at least one kind of element selectedfrom a group consisting of In, P and As is preferable because softnessis provided and it is possible to relieve the strain and to make itdifficult for crystal defects or transition to occur.

Because at least the foregoing substrate side of the foregoing bufferlayer is consisted of a semiconductor layer of high resistance, it ispossible to make it difficult for the electric current to flow.

Even in the case of a conductive type semiconductor layer which isdifferent from a conductive type semiconductor layer in which at lestthe foregoing substrate side of the foregoing buffer layer is laminateddirectly on the buffer layer, it is possible to make it difficult forthe electric current to flow to the substrate side of the buffer layer.

Because the foregoing buffer layer is formed at high temperature on ap-type low temperature buffer layer formed at low temperature on theforegoing substrate surface and such low temperature buffer layer,consisted of a high temperature buffer layer in which at least thesurface side is made an n-type, on such high temperature buffer layerare sequentially formed an n-type cald layer, an active layer, a p-typeclad layer, and a p-type contact layer in that order, the p-sideelectrode is formed on such p-type contact layer, the n-type electrodeis formed on the foregoing n-type clad layer exposed by etching or onthe high temperature buffer layer, it is possible to obtain a lightemitting element of a semiconductor of hetero junction structure inwhich it is difficult for the electric current to flow toward the bufferlayer on the substrate side.

Because the foregoing buffer layer consists of GaN, the foregoing n-typeand p-type clad layer consists of Al_(k)Ga_(1-k)N (0<k<1) respectively,the foregoing active layer consists of Ga_(y)In_(1-y)N (0<y≦1), andforegoing p-type contact layer consists of GaN, it is possible to obtaina simply structured light emitting element of a semiconductor of doublehetero junction structure.

In accordance with the light emitting element of a semiconductor of thepresent invention, because the semiconductor layer contacting at leastthe substrate of the semiconductor layer of gallium nitride typecompound to be laminated on the substrate is made a semiconductor layerwhere it is difficult for the electric current to flow, there will be nofurther increase in crystal defects or transition caused by the electriccurrent which occur due to the strain based on the difference of thelattice constant with respect to the substrate, and further the progressis restricted of crystal defects or transition toward the semiconductorlayer contributing to the light emitting portion, so that it is possibleto obtain a semiconductor layer of less crystal defects or transition.

In other words, when the electric current flows to a semiconductor layerwhere crystal defects or transition are occurring, a resistance lossoccurs and heat is generated in a portion where crysal defects ortransition are occurring, crystal defects or transition increasefurther, and the vicious cycle is repeated. On the other hand, becausecrystal defects or transition of the semiconductor of a portioncontributing to radiation results from the progress of crystal defectsor transition occurred in the semiconductor of a portion contributing toradiation results from the progress of crystal defects or transitionoccurred in the semiconductor layer contacting the substrate, byrestricting the occurrence of crystal defects or transition in thesemiconductor layer contacting the substrate, it is possible to restrictthe occurrence of crystal defects or transition in the semiconductorlayer which positively contributes to the emission of light. As aresult, a light emitting element of a semiconductor of excellent lightemitting characteristic, high reliability, and long life can beobtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory drawing illustrating a section of an example 1of the light emitting element of a semiconductor of the presentinvention;

FIGS. 2(a) through 2(c) are explanatory drawings illustratingmanufacturing processes of FIG. 1;

FIG. 3 is an energy band drawing mainly illustrating a forbidden band ofa clad layer and an active layer of an example 1 of the light emittingelement of a semiconductor of the present invention;

FIG. 4 is an explanatory drawing of a section illustrating an example 2of the semiconductor laser of the present invention;

FIGS. 5(a) through 5(c) are explanatory drawings illustrating processesof the manufacturing method of an example 2 of the semiconductor laserof the present invention;

FIG. 6 is a drawing illustrating the relation of the light emittingefficiency with respect to the thickness of a low refractive indexlayer;

FIG. 7 is an explanatory drawing of a section illustrating an example 3of the light emitting element of a semiconductor of the presentinvention;

FIGS. 8(a) through 8(c) are drawings illustrating manufacturing methodsof the light emitting element of a semiconductor of FIG. 7;

FIGS. 9(a) through 9(b) are explanatory drawings of the energy band ofthe contact layer and the electrode metal of the light emitting elementof a semiconductor of FIG. 7;

FIG. 10(a) through 10(d) are explanatory drawings illustratingmanufacturing processes of the example 4 of the light emitting elementof a semiconductor of the present invention;

FIG. 11 is a perspective drawing illustrating an example of aconventional light emitting element of a semiconductor and FIG. 12 is anexplanatory drawing of a section of a conventional semiconductor of GaAstype compound.

DETAILED DESCRIPTION

Referring now in detail to the drawings, the light emitting of asemiconductor of the present invention will be described.

EXAMPLE 1

FIG. 1 is an explanatory drawing of a section of a laser chip of amesa-type shape semiconductor of an example 1 of the light emittingelement of a semiconductor of the present invention, FIGS. 2(a) through2(c) are the manufacturing process drawing thereof, and FIG. 3 is aschematic illustration showing the energy band drawing mainly of theforbidden band of an n-type clad layer, an active layer, and a p-typeclad layer of the light emitting element of a semiconductor of theexample 1.

In FIG. 1, the numeral 1 indicates a substrate such as sapphire (singlecrystal of Al₂O₃) consisting of an n-type GaN, wherein a low temperaturebuffer layer 2 of approximately 0.01 to 0.2 μm, a high temperaturebuffer layer 3 of approximately 2 to 3 μm consisting of an n-type GaN,an n-type clad layer 14 of approximately 0.1 to 0.3 μm consisting of ann-type material such as Al_(x)Ga_(1-x)N (0≦x≦0.5, where x=0.07 forexample) which has a smaller band gap energy (forbidden band width) thana p-type clad layer, an active layer 5 of approximately 0.05 to 0.1 μmconsisting of non-doping or an n-type or a p-type material such asIn_(y)Ga_(1-y)N (0≦y≦1) which has a smaller band gap energy and largerrefractive index than both clad layers, a p-type clad layer 16 ofapproximately 0.1 to 0.3 μm consisting of a p-type Al_(z)Ga_(1-z)N(0<zμ1, 2xμz, where z=0.15 for example), and a contact layer (cap layer)7 of approximately 0.3 to 2 μm consisting of a p-type GaN are laminatedin order, a p-side electrode 8 consisting of a metal such as Au isformed on the contact layer 7, an n-side electrode 9 consisting of ametal such as Al is formed on the high temperature buffer layer 3wherein a part of the laminated semiconductor layer is removed andexposed by etching, and a current stripe is formed, so that part of thecontact layer 7 and the p-type clad layer is etched and turned into amesa-type shape, and a chip of a semiconductor laser is formed.

In the light emitting element of a semiconductor of the presentinvention, as shown in an example of this semiconductor laser, the bandgap energy of the n-type clad layer 14 smaller than the band gap energyof the p-type clad layer 16, and both clad layers 14 and 16 and theactive layer 5 is formed by a material having a greater band gap energythan that of the active layer 5.

In order to recombine the electron and the positive hole efficiently andto improve the light emitting efficiency, a light emitting element of asemiconductor of double junction structure for sandwiching the activelayer 5 consisting of a material having a small band gap energy by cladlayers consisting of material having a large band gap energy, is usedfor a semiconductor laser or an LED of high luminance. When a materialhaving a large band gap energy is used for the clad layer, effect ofconfining the electron and the positive hole increases and contributesto the emission of light without waste but the operating voltageincreases, and in practice, a material having a band gap energy whichcan ignore to a certain degree the leakage of the electron and thepositive hole from the active layer, is selected. However, the operatingvoltage increases as compared with that of the pn junction. In thepresent invention, it is so designed that it is possible to lower theoperating voltage while maintaining this degree with which the electronand the positive hole can be ignored. In other words, the effective massof the positive hole is as approximately three times large as theeffective mass of electron, and the leakage from the positive hole issmaller than that from the electron even if the band gap energy issmall. For this reason, by using for the n-type clad layer a materialhaving a band gap energy smaller than that of the p-type clad layer, itis possible to inject the electron into the active layer at low voltageand prevent the leakage of the positive hole from the active layer.

Referring now to FIG. 3 which illustrates a schematic diagram showingthe energy band drawing of the foregoing semiconductor laser of FIG. 1,the action of the present invention will be described. In FIG. 3, theletter V represents the valence band, F the forbidden band, and C theenergy band of the conduction band respectively, and the letters Arepresents the high temperature buffer layer consisting of an n-typeGaN, B the n-type clad layer 14 consisting of n-type GaN, B the n-typeclad layer 14 consisting of an n-type Al_(0.07)Ga_(0.93)N, D the activelayer 5 consisting of In_(y)Ga_(1-y)N, G the p-type cald layer 16consisting of Al_(0.15)Ga_(0.85)N, and J the contact layer 7 consistingof a p-type GaN respectively of the ranges of the energy bands thereof.

In the semiconductor laser of this example, as shown in FIG. 3, the bandgap energy of the n-type clad layer indicated by B is formed to besmaller than the band gap energy of the p-type clad layer indicated byG. The band gap energy indicated by the broken line B₁ illustrates thecase of the band gap energy which is the same as the p-type clad layerin a conventional structure.

With this structure, when voltage is applied between the p-sideelectrode 8 and the n-side electrode 9, the electron E flows from then-type GaN (high temperature buffer layer A) to the p-side, and theninto the conduction band K₁ of the active layer. In this case, becausethe band gap energy of the n-type clad layer is low, the electron E islikely to flow into the conduction band K₁ of the active layer, so thatthe electron is supplied to the active layer even with low voltage. Theelectron E flowed into the conduction band K₁ of the active layer ispulled by the p-side electrode, but is confied in the active layerbecause the band gap energy of the p-type clad layer is large. On theother hand, the positive hole flows from the p-type GaN (contact layerJ) to the n-side, and then into the valence band K₂ of the active layer.The positive hole H flowed into the valence band K₂ of the active layeris pulled by the n-side electrode, but the effective mass of thepositive hole H is as approximately three times large as the effectivemass of electron, the positive hole cannot climb over the band gapenergy even if the band gap energy of the n-type clad layer B is low,and is confined effectively in the valence band of the active layer. Asa result, recombination of the electron and the positive hole is carriedout in the active layer efficiently, and high light emitting efficiencycan be obtained.

As described above, in accordance with the present invention, becauseeach semiconductor layer is selected so that the band gap energy of then-type clad layer is made smaller than that of the p-type clad layer, itis possible to inject the electron into the active layer with lowvoltage and improve the light emitting efficiency without increasing thereactive current. The degree that makes the band gap energy of then-type clad layer smaller than that of the p-type clad layer isdetermined by the band gap energy of the active layer, and the degreemay be as low as ⅓ to ½ in the case of the p-type layer by thedifference of the band gap energy of the active layer.

In order to reduce the band gap energy by using a semiconductor ofgallium nitride type compound consisting of a general formulaAl_(p)Ga_(1-p-q)N (0≦p<1, 0<q≦1, 0<p+q≦1), it is possible to obtain asmall band gap energy by making p small, that is, by making thecomposition ratio of Al small, or by making p+q small, that is, bymaking the composition ratio of In large. For this reason, by adjustingthe composition ratio of Al and In so as to make the band gap energy ofthe clad layer larger than that of the active layer, and so as to makethe band gap energy of the n-type clad layer smaller than that of thep-type clad layer, it is possible to obtain a semiconductor layer of adesired band gap energy.

Because the example shown in FIG. 1 refers to a semiconductor, it isnecessary to confine the light in the active layer and oscillate it, sothat the refractive index of the clad layer is made smaller than that ofthe active layer, but it is not always necessary to do so in the case ofLED. However, if the composition ratio of Al is made large by theforegoing composition ratio, the refractive index is made small.

Next, by referring to FIG. 2(a) through 2(c), the manufacturing methodof the semiconductor laser shown in FIG. 1 will be described.

First, as shown in FIG. 2(a), on a substrate 1 consisting of sapphireand the like is grown by the MOCVD method, approximately 0.01 to 0.2 μmof a low temperature buffer layer 2 consisting, for example, of asemiconductor of gallium nitride type compound such as the n-type GaN,and approximately 2 to 5 μm of a high temperature bufrer layer 3consisting of the n-type GaN of the same composition is formed at 700°to 1200° C.

Next, TMI is supplied further, and approximately 0.1 to 0.3 μm of ann-type clad layer 14 consisting, for example, of the n-typeAl_(x)Ga_(1-x)N (0≦x≦0.5, where x=0.07 for example) is formed.Afterward, TMI is supplied instead of TMA and an active layer 5consisting of a non-doping type or an n-type or p-type In_(y)Ga_(1-y)N(0≦y≦1, where y=0.06 for example) is caused to grow to a thickness ofapproximately 0.05 to 0.1 μm. Then, by using the same material gas asthe material gas which is used to form the n-type clad layer 14, TMA issupplied to a reaction tube at the flow rate of 20 to 100 sccm which isapproximately twice the case of the n-type clad layer 14 so as to formapproximately 0.1 to 0.3 μm of the p-type Al_(z)Ga_(1-z)N (0<z≦1, 2x≦z,where z=0.15 for example) which is the p-type clad layer 16. Further,the same material gas used for forming the buffer layer 3 is supplied soas to form approximately 0.3 to 2 μm of a contact layer 7 consisting ofthe p-type GaN.

In order to form the foregoing buffer layer 3 or clad layer 14 into ann-type clad layer, Si, Ge, Tc is mixed into a reaction oven as theimpurity material gas such as SiH₄, GeH₄, and TeH₄, and in order to formthe clad layer 6 or the contact layer 7 into a p-type clad layer, Mg orZn is mixed into the material gas as the organometallic gas of Cp₂Mg orDMZn. However, in the case of the n-type, even if the impurity is notmixed, N is easily evaporated during film forming and is turned into then-type naturally, and therefore such nature may be utilized.

Afterwards, a protective film 10a such as SiO₂ or Si₃N₄ is provided overthe entire surface of the grown layer of a semiconductor layer (refer toFIG. 2(b)), annealed for approximately 20 to 60 minutes at 400 to 800°C., and the p-type clad layer 16 and the cap layer. 7 which are thep-type layer.

When annealing is completed, as shown in FIG. 2(C), a mask such as aresist film 10b is provided and the laminated semiconductor layer isetched until the n-type clad layer 14 or the n-type high temperaturebuffer layer 3 is exposed. This etching is carried out by the reactiveion etching under the atmosphere of a mixed gas of Cl₂ and BCl₃ forexample.

Then, a metallic film such as Au and Al is formed by sputtering, on thesurface of the laminated compound of semiconductor layer is formed thep-side electrode 8 to be electrically connected to the p-type layer, onthe surface of the exposed high temperature buffer layer 3 is formed then-side electrode 9 to be electrically connected to the n-type layer, andpart of the contact layer 7 and the p-type clad layer 16 are etched intomesa-type shape layers (refer to FIG. 1).

Next, each chip is diced, and thus semiconductor laser chips are formed.

In this example, the semiconductor laser of mesa-type shape currentstripe structure is described, but the present invention can also beapplied to a semiconductor laser of various structures such as the flushtype current limiting layer or the light emitting element ofsemiconductor which uses a semiconductor of gallium nitride typecompound such as LED of double hetero junction structure.

In accordance with the light emitting element of a semiconductor of thisexample, because the semiconductor material is selected so that it ispossible to make the band gap energy of the n-type clad layer smallerthan the band gap energy of the p-type clad layer, reactive current isless, and, it is possible to emit the light of high luminance with lowoperating voltage and to obtain a light emitting element ofsemiconductor having high light emitting efficiency.

EXAMPLE 2

FIG. 4 is an explanatory drawing of a section of the chip of asemiconductor laser which is the other example of the light emittingelement of the semiconductor of the present invention, and FIG. 5(a)through 5(c) are an explanatory drawing of a section of the process ofthe manufacturing method thereof.

As shown in FIG. 4, in the semiconductor laser of this example, a lowbuffer layer 22 of approximately 0.01 to 0.2 μm which is a lowerresistance layer with small electric resistance consisting of an n-typeAl_(t)Ga_(1-t-u)N (0≦t<1, 0<u≦1, 0<t+u≦1) and the like provided on asubstrate 1 such as sapphire (single crystal of Al₂O₃), a hightemperature buffer layer 23 of approximately 2 to 5 μm, an n-type cladlayer 24 of approximately 0.1 to 0.3 μm which is low refractive indexlayer consisting of an n-type Al_(r)Ga_(s)In_(1-r-s)N (0≦t<r<1, 0<s<1,0<r+s<1) and having the refractive index smaller than that of theforegoing buffer layers 22 and 23 (the ratio of Al is larger than thatof t), an active layer 25 of approximately 0.05 to 0.1 μm consisting ofAl_(m)Ga_(n)In_(1-m-n)N (0<m<r, 0<m+n<r+s) of a non-doping type or ann-type or a p-type and having a smaller band gap energy and a largerrefractive index than the n-type clad layer 24 (the ratio n of Al issmall and the ratio 1-m-n of In is large), a p-type clad layer 26 ofapproximately 0.1 to 0.3 μm which is a low refractive index layer of thesame composition as the n-type clad layer 24, a current limiting layer20 consisting of GaN and the like formed with a stripe groove, and acotnact layer consisting of the p-type Al_(t)Ga_(u)In_(1-t-u)N (0≦t<1,0<u≦1, 0<t+u≦1) of approximately 3 to 2 μm which is a low refractiveindex layer of the same composition as the buffer layers 22 and 23 arelaminated in order, the p-side electrode 8 is provided on the surface ofthe contact layer 27, the n-side electrode 9 on the high temperaturebuffer layer 23 which is exposed by etching a part of the laminatedsemiconductor layer, and a chip of the semiconductor laser of thisexample is formed.

In the semiconductor laser of this example, the n-type layer and thep-type layer which hold the active layer 25 is separated into the bufferlayers 22 and 23 which are the low resistance layer respectively, then-type clad layer 24 which is the low refractive index layer, the p-typeclad layer 6 which is the low refractive index layer, and the contactlayer 27 which is the low resistance layer, and the low refractive indexlayer is formed into a minimum thickness necessary to confine the lightinto the active layer. In order to inspect the influence on the lowrefractive index layer by the thickness, the inventor of the presentinvention inspected the light emitting efficiency in a manner as shownin an example of concrete structure to be described layer, namely, onthe buffer layer 23 is formed approximately 3 μm of the n-type GaN, onthe n-type and p-type clad layers 24 and 26 are formedAl_(0.3)Ga_(0.7)N, and on the contact layer 27 is formed approximately 2μm of the p-type GaN respectively, the thickness of the clad layers 24and 26 are changed differently from 0 to 0.4 μm (the thickness of thebuffer layer 3 and the contact layer 27 is also changed every time so asto keep the total thickness of the clad layers 24 and 26 constant), andthus the light emitting efficiency is inspected. Results of theinspection is shown in FIG. 6. As is apparent from FIG. 6, it isnecessary that the thickness of the low refractive index layer is formedto be 0.05 to 0.3 μm, and preferably to be approximately 0.1 to 0.2 μm,and is formed to be 10 to 50% with respect to the entire thickness ofthe n-type layer or the p-type layer, and preferably to be approximately10 to 30%.

In case where a semiconductor of gallium nitride type compound isexpressed as the general formula of Al_(p)Ga_(q)In_(1-p-q)N, in order tomake the refractive index small in a semiconductor of gallium nitridetype compound, it is necessary to make the composition ratio of Al largeand to make the composition ratio of In small, and when the compositionratio of Al is made large, the resistance is increased and theresistance loss results. Therefore, in this example, a low refractiveindex layer (the n-type clad layer 24 and the p-type clad layer 26) isprovided for confining the light into the semiconductor portion adjacentto the active layer 25 of the n-type layer (the buffer layers 22 and 23,and the n-type clad laeyr 24) which hold the active layer 25therebetween, and other portion is made a low resistance where the lossof resistance does not occur.

In order to make the specific resistance small in the semiconductor ofgallium nitride type compound expressed as the foregoing general formulaof Al_(p)Ga_(q)In_(1-p-q), the composition ratio of Al is to be simplymade small. In this case, it is necessary for the n-type layer and thep-type layer which hold the active layer therebetween to be providedwith the function as the clad layer to confine the light into the activelayer, it is preferable that even the low resistance layer (the bufferlayers 22 and 23 and the contact layer 27) is formed by a material whichhas smaller refractive index and larger band gap energy than the activelayer 25. In order to achieve this object, by making the compositionratio of In large in the active layer 25 and by making such compositionratio small in the n-type layer and the p-type layer, it is possible toobtain a layer which is of low resistance has larger band gap energy andsmaller refractive index than the active layer 25.

In this example, the composition ratios of the n-type buffer layers 22and 23 and the contact layer 27 are made to be the same, and thecomposition ratios of the n-type clad layer 24 and p-type clad layer 26are made to be the same, but respective composition ratios may notalways have to be the same, and the low refractive index layer (then-type clad layer 24 and p-type clad layer 26) may have smallerrefractive index and larger band gap energy than the active layer 25 orthe low resistance layer (the buffer layers 22 and 23 and the contactlayer 27), and the low resistance layer may be of a material havingsmaller specific resistance than the low refractive index layer.

The low temperature buffer layer 22 is made to be a low resistance layerin this example, but it is not necessary to make the low temperaturebuffer layer to be an electric current path if it is possible to providethe *n-side electrode 9 on the high temperature buffer layer 23, so thatthe low temperature buffer layer 22 may be made to be a high resistancelayer or the p-type layer. In this case, it is preferable that electriccurrent is not flowed to the low temperature buffer layer where crystaldefects or transition are likely to occur, thereby making it possible toprevent an increase of crystal defects or transition.

Further, in the boundary between the low resistance layer and the lowrefractive index layer, the composition may not be changed sharply butmay be changed gradually. Causing the composition to change gradually iseffective to restrict the strain generating in the interface due tosudden change in composition. In addition, a second clad layer which isa low resistance layer similar to the contact layer 27 may be providedbetween the p-type clad layer 26 and the contact layer 27, so that thehigh temperature buffer layer 23 accomplishes the function of the cladlayer of low resistance layer.

Next, the manufacturing method of the semiconductor laser of thisexample will be described by a definite example thereof. First, as shownin FIG. 5(a), a substrate 1 consisting of sapphire and the like isinstalled in a reaction tube, and in the same manner as the example 1,10 slm of the carrier gas H₂, 100 sccm of the reactant gas TMG, and 10slm of NH₃ are introduced to grow in vapor phase at 400° to 700° C. bymeans of the organometallic vapor phase growth method (hereinafterreferred to as the MOCVD method), and a low temperature buffer layer 22which is a polycrystalline film consisting of GaN of approximately 0.01to 0.2 μm is formed. Then, by raising the temperature to 700° to 1200°C. and allwoing to stand for approximately 5 to 15 minutes, polycrystalline of the low temperature 5 to 15 minutes, polycrystal line of the lowtemperature buffer layer 2 is made into a single crystal, by introducingthereon the same material gas as the foregoing and causing it to reactin vapor phase at high temperature of 700° to 1200° C., a hightemperature buffer layer 23 consisting of single crystal of GaN isformed to 2 to 5 μm thick.

Further, by mixing TMA at the flow rate of 10 to 200 sccm and causing itto react in vapor phase, an n-type clad layer 24 consisting ofAl_(0.2)Ga_(0.8)N is formed to 0.1 to 0.3 μm thick.

Next, the dopant SiH₄ is stopped and in place of TMA, TMI is supplied atthe flow rate of 10 to 200 sccm so as to form approximately 0.05 , to0.1 μm of a non-doping active layer 5 consisting of In_(0.1)Ga_(0.9)N,further the material gas of the same composition as that of the n-typeclad laeyr 24 is supplied, the impurity material gas is replaced by SiH₄and Cp₂Mg or DMZn is introduced at the flow rate of 10 to 1000 sccm soas to reform the p-type clad layer 6 consisting of Al_(0.2)Ga_(0.8)N to0.1 to 0.3 μm thick, and then, the n-type GaN layer is formed toapproximately 0.1 to 0.5 μm thick so as to form a current limiting layer20 by supplying TMG, NH₃, and SiH₄.

Afterward, the furnace temperature is lowered to approximately the roomtemperature, a substrate with semiconductor layer laminated thereon istaken out from the MOCVD device, and as shown in FIG. 5(b), a stripegroove is formed by etching by lithographic process, and the currentlimiting layer 20 is formed.

Afterwards, as shown in FIG. 5(c), the substrate is put into the MOCVDdevice again with the temperature set at 700° to 1200° C., TMG and NH₃as the reactant gas and Cp₂Mg or DMZn as the dopant are supplied, andapproximately 2 to 3 μm of a contact layer 27 consisting of GaN isformed.

Afterwards, a protective film such as SiO₂ and Si₃N₄ is provided on theentire surface of a semiconductor layer and annealed for approximately20 to 60 minutes at 400° to 8000° C. so as to activate the p-type cladlayer 26 and the contact layer 27.

Next, in order to form the n-side electrode, a mask is formed by aresist film and the like and part of a semiconductor layer laminatedunder atmosphere of Cl₂ gas is provided with reactive ion etching, ahigh temperature buffer layer 23 which is an n-type layer is caused tobe exposed, a p-side electrode 8 consisting of Au and Au—Zn and the likeis formed on the contact layer 27, an n-side electrode 9 consisting ofAu and Au—Zn and the like is formed on the contact layer 27, an n-sideelectrode 9 consisting of Al and Au-Gc and the like is formed on thehigh temperature buffer layer 23, and a chip of a semicondcutor laser isformed by dicing (refer to FIG. 4).

In this example, the semiconductor laser is a semiconductive laser ofrefractive index wave guide type provided with the current limitinglayer 20, but it is also the same in the case of a gain wave guidestripe type semiconductor laser. Further, it is the semiconductor laserwhich uses a semiconductor of gallium nitride type compound whosegeneral formula is expressed as Al_(p)Ga_(q)In_(1-p-q)N, and though itis not so remarkable as the semiconductor of gallium nigride typecompound, it will be effective by application of the present inventioneven in the case of a semiconductor of other compound such as thesemiconductor of arsenic gallium type compound.

In accordance with a semiconductor laser of the example 2, because then-type layer and the p-type layer for holding the active layertherbetween are respectively formed by at least a low refractive indexlayer and a low resistance layer, confinement of the light into theactive layer is carried out by the low refractive index layer, electricresistance of the portion composing other electric current path becomessmall due to the low resistance layer, so that it is possible to lowerthe operating voltage. As a result, it is possible to obtain asemiconductor laser which operates with low operating voltage and hashigh light emitting efficiency.

EXAMPLE 3

In a further other example of the semiconductor laser of the presentinvention shown in FIG. 7, a low temperature buffer layer 2 ofapproximately 0.01 to 0.2 μm consisting of the n-type GaN and the likeon a substrate 1 such as sapphire (single crystal of Al₂O₃), a hightemperature buffer layer 3 of approximately 2 to 5 μm, a lower caldlayer 4 of approximately 0.1 to 0.3 μm consisting of the n-typeAl_(k)Ga_(1-k)N (0<k<1), an active layer 5 of approximately 0.05 to 0.1μm consisting of a non-doping or n-type or p-type In_(y)Ga_(1-y)N(0<y<1) and having smaller band gap energy and larger refractive indexthat the lower clad layer 4, an upper clad layer 6 of approximately 0.3to 2 μm which is of the same composition as the lower clad layer 4 andof the p-type, and a contact layer 37 consisting of the p-type GaN layer37 a of approximately 0.3 to 2 μm which is of the same composition asthe buffer layers 2 and 3 and of the p-type and the p-typeIn_(a)Ga_(1-a)N (0<a<1) layer 27 b of approximately 0.05 to 0.2 μmprovided on the surface thereof are laminated in order, a p-sideelectrode 8 is provided on the In_(a)Ga_(1-a)N layer 37 b of the surfaceof the contact layer 37, an n-side electrode 9 is provided on the n-typeclad layer 4 or the high temperature buffer layer 3 exposed by etching apart of the laminated semiconductor layer, and a chip of thesemiconductor laser of this example is formed.

In the semiconductor laser of this example, a semiconductor layer ofgallium nitride type compound is laminated, the material of the p-typecontact layer 37 to be provided with the p-type electrode 8 has smallerband gap energy than GaN and is difficult for the surface level tooccur, for example, the p-type In_(a)Ga_(1-a)N layer 37 b is provided onthe surface of the GaN laeyr 37 a, and the p-side electrode 8 isprovided on the In_(a)Ga_(1-a)N layer 37 b. Since lattice mismatchingresults if In is mixed with GaN, such mixture is not used in a layerexcept when a material of small band gap energy is indispensable as inthe case of the active layer, and there has been no conception at all touse the In_(a)Ga_(1-a)N layer in the contact layer 37. However, in thelight emitting element of a semiconductor which uses a semiconductor ofgallium nitride type compound, it is not possible to increase thecarrier density of the p-type layer above a certain value, and theincrease in the contact resistance between the p-type layer and thep-side electrode increased the operating voltage and caused loweredlight emitting efficiency. As the reault of assiduous studies, theinventor of the present invention overcome the problem of latticemismatching by forming the semiconductor layer to a thickness ofapproximately 0.05 to 0.2 μm in which a film is formed even if a littlelattice mismatching should occur, found out that it is possible to lowerthe contact resistance with metal, and completed the present invention.

In the case where In_(a)Ga_(1-a)N contained with In as the material ofsmall band gap energy is used, if the composition ratio of In isincreased, the thickness for example of the foregoing semiconductor wasnot preferable in that a phenomenon such as occurrence of transitionemerged, but by setting the composition ratio a of In to 0<a≦0.5,preferable to 0.05≦a<0.3, more preferable to 0.05≦x≦0.2, it was madepossible to reduce the contact resistance with metal without causing theproblem transition.

Even in the case where GaAs or Gap was used as the material of smallband gap energy instead of In_(a)Ga_(1-a)N, it was possible to performan operation in which the contact resistance with metal was also small,it was also difficult for the surface level to occur on thesemiconductor surface, and the operating voltage was low. Growthtemperature of GaAs or GaP is different from that of the GaN type layer,but it is possible to obtain a layer by forming the GaN type layer andthen by growing it by lowering the temperature inside the MOCVD deviceto 500° to 800° C. Though lattice mismatching of GaN occurs with respectto GaAs or GaP, but the influence due to lattice mismatching is reducedto certain extent by setting the foregoing thickness to approximately0.05. to 0.2 μm.

By mixing In further with GaAs or GaP, it is possible to use thecharacteristic that it is easy to make alloy using a metal which isdifficult to be oxidized than Al and Ga, and it was possible to reducethe contact resistance further. In this case, it is possible to increasethe composition ratio of In to approximately 0 to 0.5.

Next, referring to FIG. 9(a) and FIG. 9(b), and by providing asemiconductor of small band gap energy on the surface of the contactlayer 37, the principle that the contact resistance with the p-sideelectrode is reduced will be described.

FIG. 9(a) and FIG. 9(b) are the drawings showing the energy band of thecontact layer 37 and the p-side electrode 8, in which the left side ofthe drawing indicates the side of the p-type clad layer 6 of the contactlayer 37, the right thereof indicates the side of the p-side electrode8, and the letter L represents the energy band of the GaN layer 37 a, Mrepresent that of the In_(x)Ga_(1-x)N layer 37B, and N represent that ofpart of the p-side electrode 8 respectively. FIG. 9(a) and FIG. 9(b)typically illustrate the state where the energy levels move upward anddownward on the surface of the GaN layer 37 a or the In_(a)Ga_(1-a)Nlayer 37 b depending on the composition of the semiconductor layer andthe kind of, the metal for electrode to be provided on the surface ofthe semiconductor layer, and indicate the same phenomena in any state ofFIG. 9(a) and FIG. 9(b). In FIG. 9(a) and FIG. 9(b), P₁ and P₂ indicatethe valence band of GaN and In_(a)Ga_(1-a)N respectively, Q₁ and Q₂ theconduction band respectively, and R the energy level where the electronof an electrode metal is maximum. The gap F₁ of P₁ and Q₁ and the gap F₂of P₂ and Q₂ respectively represent the band gap energy (forbidden band)of GaN and In_(a)Ga_(1-a)N. The flow of electric current from the p-sideelectrode to the contact layer means that the positive hole moves fromthe energy level R of the electrode metal to the valence band P₁ of GaN,but in accordance with the present invention, because theIn_(a)Ga_(1-a)N layer 12 is provided, it is normal once the positivehole climbs over the gap energy E_(v1) and flows to the valence band P₂of the In_(a)Ga_(1-a)N layer 12, and then climbs over the gap evergyE_(v2) from P₂ to P₁ and flows, so that the positive hole flows easilybecause it is not neecssary for the positive hole to climb over the gapenergy E_(v) at one time when there is no In_(a)Ga_(1-a)N. In this case,assuming that the constant of the electric current including the item ofthe temperature and k₁ and k₂ respectively, the gap energy can beexpressed by exp {⁻(k₁E_(v1) ⁺k₂E_(v2))]. The reason that the energybarrier is devided into two stages is that the forbidden band width F₂of In_(a)Ga_(1-a)N is smaller than the forbidden band width F₁ of GaN,and ideally, it is desirable to use the material of the forbidden bandwidth F₁ of GaN GaAs or GaP or In_(b)Ga_(1-b)As or In_(b)Ga_(1-b)P hasalso the same relation of the forbidden band width, and the contactresistance is reduced in the same manner.

Next, referring to FIGS. 8(a) through 8(c), the manufacturing method ofthe semiconductor laser of this example shown in FIG. 7 will bedescribed.

First, as shown in FIG. 8(a), to the substrate 1 consisting of sapphireand the like in the same manner as the example 1 is supplied by theMOCVD method the carrier gas H₂ together with TMG and NH₃ which are theorganometallic compound gas, and SiH₄ as the dopant, and the lowtemperature buffer layer 2 and the high temperature buffer layer 3consisting of gallium nitride type semiconductor layer such as then-type GaN layer are grown respectively to approximately 0.01 to 0.2 μmand 2 to 5 μm.

Then, TMA is added further to the foregoing gas, and n-type clad layer 4containing Si and the like of the n-type dopant as the SiH₄ gas and thelike is formed to approximate thickness of 1 to 2 μm.

Next, as a material in which the band gap energy is smaller than that ofthe clad layer, in place of the foregoing material gas for example, TMIis introduced and approximately 0.05 to 0.1 μm of an active layer 5 isformed, further, in place of SiH₄, the same material gas used forforming the n-type clad layer 4 is used and Cp₂Mg or DMZn is introducedinto a reaction tube as the p-type impurity, and the p-type GaN layerwhich is the p-type clad layer 6 is caused to grow in vapor phase.

Then, as shown in FIG. 8(b), in order to form a contact layer, Cp₂Mg orDMZn is supplied as the dopant gas using the same gas of the foregoingbuffer layer 3, and the p-type GaN layer 37 a is grown to a thickness ofapproximately 0.3 to 2 μm.

Further, in order to reduce the contact resistance with the p-sideelectrode, TMI is added to the same material gas as the foregoing GaNlayer 37 a, and the In_(a)Ga_(1-a)N (0<a<1, where a=0.1 for example) isformed to a thickness of approximately 0.05 to 0.2 μm. If theIn_(a)Ga_(1-a)N layer is excessively thick, the resistance of the filmitself influences the entire layer, and if it is excessively thin, thecontact resistance cannot be reduced.

In the foregoing description, the p-type In_(a)Ga_(1-a)N layer is usedas part of the contact layer, but by changing the gas to the p-typeIn_(a)Ga_(1-a)N, it is possible to obtain the same effect by forming thep-type GaAs, p-type GaP, p-type In_(b)Ga_(1-b)As (0<b<1, where b=0.2 forexample) or p-type In_(b)Ga_(1-b)P (0<b<1, where b=0.5 for example) asthe contact layer on the side contacting the p-side electrode. In thiscase, it is possible to obtain the same effect by lowering the insidetemperature of the MOCVD device to 500° to 800° C. and introducing thegas, in place of the foregoing TMI, or introducing TMI and tertiarybutyl arsine (TBA) or tertiary butyl phosphine (TBP).

Afterwards, a protective film such as SiO₂ and Si₃N₄ is provided overthe entire surface of the grown layer of a semiconductor, annealing orelectron irradiation is provided for approximately 20 to 60 minutes at400° to 800° C. so as to activate the p-type clad layer 6 and thecontact layer 37. Upon completion of annealing, the protective film isremoved by wet etching.

Then, in order to form an electrode on the n-side, resist is applied andpatterning is provided, a resist film is provided on the surface of asemiconductor layer of gallium nitride type compound from which theprotective film is removed as shown in FIG. 8(c), part of thesemiconductor layer is removed by dry etching, the high temperaturebuffer layer 3 which is an n-type layer or the n-type clad layer 4 isexposed, on the surface of the exposed high temperature buffer layer 3(or the n-type clad layer 4) is formed an n-side electrode 9 consistingof Al and the like to be electrically connected to the n-type layer, andon the surface of the contact layer 37 of a semiconductor layer of thelaminated compound is formed a p-side electrode 8 consisting of ametallic film such as Au and Zn respectively by sputtering and the like.Next, part of the contact layer 7 and the p-type clad layer is turnedinto the mesa-type shape by etching, and a semiconductor laser chip isformed by dicing each chip.

And, a structure for lowering the contact resistance between the p-sideelectrode 8 and the contact layer 37 of this example can be applied tothe light emitting element of various semiconductors such as an LED ofdouble hetero junction or an LED of pn junction.

In accordance with the light emitting element of the semiconductor ofthe example 3, because the portion at least contacting the p-sideelectrode of the contact layer of the p-side electrode is formed with asemiconductor material having smaller band gap energy than the p-typeGaN, it is possible to reduce the influence of the surface level and itis also possible to reduce the contact resistance with the p-sideelectrode. Therefore, it is possible to reduce the operating voltage andimprove he light emitting efficiency.

EXAMPLE 4

In the light emitting element of the semiconductor of the example, whenforming a semiconductor of gallium nitride type compound on a substratesuch as sapphire, a semiconductor layer in which In, P or As iscontained is provided on a semiconductor layer at least contacting asubstrate, the strain of such semiconductor layer is relieved so as torestrict occurrence of crystal defects or transition involved in latticemismatching with a substrate such as sapphire, thereby preventing theprogress of crystal defects or transition toward the semiconductor layerwhich contributes to the light emitting portion.

A semiconductor layer containing In is one of which part of GaN isreplaced by In and by forming on the substrate as a semiconductor layerof In_(c)Ga_(1-c)N (0<c<1), In becomes heavier than Ga and is movedeasily, so that it is possible to form a soft buffer layer with lessstrain in which coupling between atoms is easily cut. The compositionratio c of In is set to 0 to 1, and preferable to 0.1 to 0.5, and morepreferably to approximately 0.1 to 0.3. If the composition ratio of Inis excessively large, the difference between the substrate and thelattice constant becomes excessively large, posing excessively largeproblem of lattice mismatching, and therefore, the effect of relievingthe strain by In is now shown if the composition ratio of In isexcessively small. In this In_(c)Ga_(1-c)N, the same applies to the caseof In_(d)Al_(e)Ga_(1-d-e)N (0<d<1, 0<e<1, 0<d+e<1) in which part of Gais replaced further by Al.

And, in a semiconductor layer containing p or As, part of N of GaN isreplaced by P or As and formed on a substrate as GaN_(w)P_(1-w) (0<w<1)or GaN_(v)As_(1-v) (0<v<1), because p or As is heavier than N and ismoved easily, so that it is possible to form a soft buffer layer withless strain in which coupling between atoms is easily cut. Compositionratio w and V of P and As is set to 0<w and v≦0.2, preferably to 0<w andv≦0.1, and more preferably to approximately 0.02≦w and v≦0.06. When thecomposition ratio of P and As becomes excessively large, latticemismatching between GaN and N becomes large, and therefore, the effectof relieving the strain is low shown if the composition ratio of P andAs is excessively small.

In P and As, both atoms may be contained not only in one side only butmay be contained in to form of mixed crystals. In this case, it ispreferable that the total composition ratio of P and As is contained inthe range of the foregoing w or v. Further, each atom such as In and P,In and As, and In and As may form a mixed crystal respectively. In thiscase, it is possible to contain in the range of the foregoing c and tocontain P and/or As in the range of foregoing w or v.

Next, further details will be described in concrete example.

FIGS. 10(a) through 10(d) are explanatory drawings of a section of theprocess of the concrete example of the light emitting element of asemiconductor of the example 4.

First, as shown in FIG. 10(a), to a substrate 1 consisting of sapphireand the like is supplied by the MOCVD method the carrier gas, TMG, NH₃,and TMI, a low temperature buffer layre 42 consisting of a semiconductorlayer of the n-type gallium nitride type compound of In_(c)Ga_(1-c)N(0<c<1, where c=0.2 for example) is formed to approximate thickness of0.01 to 0.2 μm at low temperature of 400° to 600° C. Composition ratioof In is set preferably to 0.1 to 0.5, sand more preferably to 0.1 to0.3. If the composition ratio is excessively large, the problem oflattice mismatching results and if excessively small, the effect ofrelieving the strain is not shown.

Afterwards, in order to form a buffer layer of single crystal,temperature is at 700° to 1200° C. and the low temperature buffer layeris made into a single crystal layer, and on the surface thereof is grownthe high temperature buffer layer 3 consisting of GaN or AlGaN type orInAlGaN type and the like to approximate thickness of 2 to 5 μm.Afterwards, a clad layer 4 consisting, for example, of the n-typeAl_(k)Ga_(1-k)N (0<k<1) and active layer 5 consisting of In_(y)Ga_(1-y)N(0<y<0.2) is grown to approximate thickness 0.1 to 0.3 μm and 0.05 to0.1 μm respectively. To form the clad layer, TMG, NH and TMA, and SiH₄as the dopant are supplied, and TMI is introduced in place of TMA andcaused to react.

Further, with the same material gas as the material gas used for formingthe n-type clad layer 4, impurity material gas is replaced by SiH₄,Cp₂Mg or DMZn gas is introduced into a reaction tube as the p-typeimpurity material gas, so that the p-type Al_(k)Ga_(1-k)N (0<k<1) layerwhich is the p-type clad layer 6 is caused to grow in vapor phase.

Then, a contact layer (cap layer) 7 consisting, for example, of thep-type GaN layer is grown and formed to 0.2 to 3 μm.

Afterwards, as shown in FIG. 10(c) in the same manner as the example 1,a protective film 10 a such as SiO₂ and Si₃N₄ is provided on the entiresurface of the grown layer of a semiconductor layer, annealing orelectron irradiation is provided for approximately 20 to 60 minutes at400° to 800° C., thereby activating the p-type clad layer 6 and thecontact layer 7.

Then, in order to form an electrode of the n-side after the protectivefilm 10 a is removed, resist is applied and patterning provided, a cladlayer 4 or buffer layer 3 which are the n-type layer are exposed (referto FIG. 10(c)) by removing part of each of grown semiconductor layer byetching in the same manner as the example 1, the n-side electrode 9 tobe electrically connected to the n-type layer and the p-side eletrode 8to be electrically connected to the p-type contact layer 7 arerespectively formed on the surface of a semiconductor layer of thelaminated comopund by sputtering and the like (refer to FIG. 10(d)), andLED chips are formed by dicing.

In accordance with this example, because a semiconductor layer ofgallium nitride type compound containing In is used for the lowtemperature buffer layer 42 as the semiconductor layer which contacts asubstrate such as sapphire, the semiconductor becomes one which is softand the coupling thereof between atoms is easily cut, and it is possibleto considerably relieve the strain involved in lattice mismatching. Thelow temperature buffer layer 42 may not be have to be In_(c)Ga_(1-c)Nbut the same result was obtained by In_(d)Al_(e)Ga_(1-d-e)N.

With respect to the material gas used for forming the foregoing lowtemperature buffer layer 42, in place of TMI, tertiary butyl phosphine(TBP) or tertiary butyl arsine (TBA), for example, is introduced, asemiconductor layer of a compound consisting of GaN_(w)P_(1-w) (0<w<1)or GaN_(v)As_(1-v) (0<v<1) is formed, and the same structure andmanufacturing method were used as in the case of In_(c)Ga_(1-c)N. Thegrowth temperature of the low temperature buffer layer 24 was 400° to600° C.

As a result, because a semiconductor of gallium nitride type compoundcontaining P or As is used for the low temperature buffer layer as asemiconductor layer which contacts a substrate such as sapphire, as inthe case of the example 1 in which such semiconductor contained In, itis possible to obtain a soft semiconductor layer with coupling thereofbetween atoms being easily cut, and to considerably relieve the straininvolved in the lattice mismatching.

In each of the foregoing example, examples of LED of double heterojunction were described, but it is the same with a laser diode of pnjunction various structures.

In accordance with the light emitting element of the semiconductor ofthe example 4, because a low temperature buffer layer consisting of asemiconductor containing at least In, P or As is provided as asemiconductor of gallium nitride type compound to be formed on thesurface of substrate, the semiconductor is soft and the strain isrelieved. As a result, occurrence of crystal defects and transition inthe low temperature buffer layer is restricted, progress of crystaldefects or transition toward the semiconductor which contributes toemission of light can also be restricted, thereby improving the lightemitting characteristic, improving reliability, and extending the lifefurther.

EXAMPLE 5

In the light emitting element of the semiconductor of this example 5, abuffer layer which is a semiconductor layer contacting a substrate of asemiconductor layer of gallium nitride type compound containing at leastan n-type layer and a p-type layer to be laminated to form a lightemitting portion on a substrate such as sapphire is composed by asemiconductor in which it is difficult for electric current to flow. Inother words, because the lattice constant of the semiconductor layer ofgallium nitride type compound and that of the sapphire substrate aredifferent in that the former is 4.758Å and the latter is 3.189 Å, straindue to lattice mismatching occurs in the buffer larer on the substrate,and crystal defects or transition are likely to occur. When electriccurrent flows into the semiconductor where such crystal defects ortransition are occurring, heat is generated and crystal defects ortransition increase in the portion where cyrstal defects or transitionoccur. Because the crystal defects or transition occurred in this bufferlayer progress toward the semiconductor layer which forms the lightemitting portion, by preventing electric current form flowing to thebuffer layer on the substrate as much as possible, it is possible torestrict the occurrence of crystal defects or transition in the entiresemiconductor layer.

In order to make the buffer layer portion contacting the substrate as alayer where electric current is difficult to flow, it is possible toobtain such layer by making the layer as a high resistance layer or alayer of opposite conduction type by introducing a semiconductor layerof the upper part of the buffer layer and opposite conduction typedopant when growing a semiconductor layer in vapor phase. For example,because it is necessary to anneal and activate the p-type layer whenforming the light emitting element a semiconductor by laminating asemicondcutor of gallium type compound, the lower portion which is thesubstrate side is made into an n-type layer and the surface side is madeinto a p-type layer, and then are laminated. When growing asemicondcutor of gallium nitride type compound in vapor phase, N of thesemiconductor of gallium nitride type compound is easily evaporated, sothat the n-type layer is obtained without mixing a dopant. Therefore,when forming a buffer layer, by mixing the n-type clad layer to beformed on the buffer layer and the p-type dopant of opposite conductiontype, the layer to be originally formed into an n-type layer isneutralized by the p-type dopant and becomes a high resistance layer,and becomes a p-type layer by mixing more p-type dopant. Because then-side electrode is provided on the surface of the n-type layer of thebuffer layer or the clad layer, electric current does not flow to thehigh resistance layer of the substrate side or to the p-type layer. As aresult, electric current does not flow to the buffer layer of thesubstrate side in particular where crystal defects or transition arelikely to occur, so that it is possible to prevent an increase ofcrystal defects or transition.

When a semiconductor of gallium nitride type compound containing atleast either In, P or As is used for the foregoing buffer layer, In isheavier than Ga and p or As is heavier than N, it is preferable that itis easy to relieve the strain because the coupling between atoms iseasily cut, and it is further possible to prevent an increase of crystaldefects or transition.

Next, the manufacturing method of the light emitting element of thesemiconductor of the example 5.

First, as shown in FIG. 10(a) of the example 4, on a substrate 1consisting of sapphire and the like is supplied by the MOCVD method thecarrier gas H₂ together TMG at the flow rate of 20 to 200 sccm, NH₃, atthe flow rate 5 to 20 slm, and Cp₂Mg or DMZn at the flow rate of 10 to1000 sccm so as to grow in vapor phase at 400 to 700° C., and a lowtemperature buffer layer 42 consisting of GaN having the specificresistance of approximately 10 to 10¹⁸·cm is grown to approximatethickness of 0.1 to 0.2 μm .

Then, the temperature is raised to approximately 700° to 1200° C. tomake the foregoing low temperature buffer layer 42 into single crystal,and the same material gas as the foregoing gas is continuously supplied,the dopant gas is changed to SiH₄ and the high temperature buffer layer3 consisting of the n-type GaN is formed to approximate thickness of 2to 5 μm. When growing the high temperature buffer layer 3, it ispossible to obtain the n-type layer as described above even if the layeris grown without supplying the dopant gas, it is preferable to supplythe dopant gas in order to sufficiently increase the carrier gasdensity.

Afterwards, in the same manner as the example 4, semiconductors of then-type clad layer, active layer, p-type clad layer, and contact layerare laminated, annealed, and etched to be formed into the p-sideelectrode and the n-side electrode, and then made into chips.

In this example, the flow rate of the p-type dopant supplied duringgrowth of the low temperature buffer layer 42 is set to approximately 10to 100 sccm and the buffer layer is formed as a high resistance layerhaving the specific resistance of approximately 1000 to 10¹⁸·cm, but bysetting the flow rate of the p-type dopant to approximately 500 to 1000sccm and forming the layer as the p-type layer, the high temperaturebuffer layer 3 thereon is the n-type and the n-side electride isprovided on the n-type layer, therefore, little electric current flowsto the p-type layer on the insulated substrate, and it is possible toform a layer where electric current is difficult to flow.

In addition, in this example, the entire high temperature buffer layer 3on the low temperature buffer layer 42 is formed by the example of then-type layer, but it is also possible to make the lower layer side ofthe high temperature buffer layer 3 as the high resistance layer or thep-type layer. In this case, by changing only the dopant gas to besupplied while growing a semiconductor layer in vapor phase, it ispossible to change the conduction type dopant. Further, the entire ofthe high temperature buffer layer 3 may be made a high resistance layeror a p-type layer and the n-type clad layer may be made to a thicknessto an extent where the series resistance does not pose a problem.

Further, in the light emitting element of a semiconductor which uses asemiconductor of gallium nitride type compound, as stated above,normally the n-type layer is formed n the lower layer side close to thesubstrate and p-type layer is formed on the surface side, but even ifthe n-type layer and the p-type layer are formed in the opposite way, itis possible to form a layer where the same electric current is difficultto flow by simply reversing the n-type layer and the p-type layer.

In addition, in this example and LED of double hetero juction isdescribed, the present invention can be applied to the light emittingelement of semiconductor such as laser diode having LED of pn junctionor various structures.

In accordance with the light emitting element of the semiconductor ofthe example 5, because the semiconductor layer of gallium nitride typecompound (buffer layer) contacting the substrate where crystal defectsor transition are likely to occur due to the lattice mismatching is madea layer where electric current is difficult to flow, it is possible toprevent an increase of crystal defects or transition caused by electriccurrent. As a result, it is possible to restrict the occurrnce ofcrystal defects or transition in the semiconductor layer composing thelight emitting portion, and it is possible to obtain the light emittingelement of semiconductor having excellent light emitting characteristic.

Further, crystal defects or transition do not increase during operationdue to the influence of electric current, while the reliability isimproved and the life is extended as well.

In each example of the foregoing examples 1 through 5, examples ofsemiconductor layers of special compositions are described as thesemiconductor layer of gallium nitride type compound, but not limitingto the material of the foregoing composition, the semiconductor layer isgenerally consisted of Al_(p)Ga_(1-p-q)N (0≦p<1, 0<q≦1, 0<p+q≦1), theratio of each composition may be selected so that the bond gap energy ofthe active layer, for example, is smaller than the band gap energy ofthe clad layer, or the composition ratios p and q may be changed so asto satisfy the band gap energy or the refractive index of eachsemiconductor layer according to the desired light emitting element ofsemiconductor. The present invention can be applied in the same manneras to the material in which part or whole of N of the foregoingAl_(p)Ga_(1-p-q)N is replaced by As and/or P.

1-21. (canceled)
 22. A gallium nitride (GaN) type semiconductor lightemitting device comprising: an active layer including at least one InGaNlayer, the active layer having a thickness not greater than 0.1micrometers; a first electrode electrically connected to the activelayer via a first layer, the first layer having at least one layerincluding Al_(x)Ga_(1-x)N where 0≦x<1; and a second electrodeelectrically connected to the active layer via a second layer, thesecond layer having at least one layer including Al_(z)Ga_(1-z)N where0<z≦1 and x<z.
 23. The semiconductor light emitting device according toclaim 22, where 2x<z.
 24. The semiconductor light emitting deviceaccording to claim 22, wherein at least one of the first layer and thesecond layer includes In.
 25. The semiconductor light emitting deviceaccording to claim 22, wherein a band gap energy of each of the firstlayer and the second layer is respectively greater than a band gapenergy of the active layer, and the band gap energy of the first layeris less than a band gap energy of the second layer.
 26. Thesemiconductor light emitting device according to claim 25, wherein adifference between the band gap energy of the first layer and the bandgap energy of the active layer is not less than one third of adifference between the band gap energy of the second layer and the bandgap energy of the active layer.
 27. The semiconductor light emittingdevice according to claim 22, wherein leakage of a positive hole fromthe active layer is prevented.
 28. The semiconductor light emittingdevice according to claim 22, wherein an operating voltage to inject anelectron into the active layer of the device is lower than it would beif z were equal to x for the same x.
 29. The semiconductor lightemitting device according to claim 22, wherein the first layer includesat least one non-doped n-type layer.
 30. A gallium nitride (GaN) typesemiconductor light emitting device comprising: an active layerincluding at least one InGaN layer, the InGaN layer having a thicknessnot greater than 0.1 micrometers; an n-side electrode electricallyconnected to the active layer via an n-side layer, the n-side layerincluding at least one GaN layer that is one of doped and non-doped; anda p-side electrode electrically connected to the active layer via ap-side layer, the p-side layer including at least one GaN layer and atleast one of an AlGaN layer.
 31. The semiconductor light emitting deviceaccording to claim 30, wherein the active layer has a thickness of about0.1 micrometers.
 32. The semiconductor light emitting device accordingto claim 30, wherein the p-side layer includes at least one GaN layerand at least one Al_(z)Ga_(1-z)N layer where 0<z<0.15.
 33. Thesemiconductor light emitting device according to claim 30, wherein aband gap energy of each of the n-side layer and the p-side layer isgreater than a band gap energy of the active layer, and the band gapenergy of the n-side layer is less than a band gap energy of the p-sidelayer.
 34. The semiconductor light emitting device according to claim33, wherein a difference between the band gap energy of the n-side layerand a band gap energy of the active layer is not less than one third ofa difference between the band gap energy of the p-side layer and theband gap energy of the active layer.
 35. The semiconductor lightemitting device according to claim 30, wherein leakage of a positivehole from the active layer is prevented.
 36. The semiconductor lightemitting device according to claim 30, wherein an operating voltage toinject an electron into the active layer is lower than it would be if,with the same n-side layer, the p-side layer contained substantially thesame composition as that of the n-side layer.
 37. A GaN typesemiconductor light emitting device comprising: a light-emitting layer;an n-side electrode electrically connected to the light-emitting layerthrough an n-type contact layer and an n-side layer, the n-side layerincluding Al and/or In; and a p-side electrode electrically connected tothe light-emitting layer through a p-type contact layer and a p-sidelayer, the p-side layer including Al and/or In, wherein a band gapenergy of the n-side layer is smaller than a band gap energy of thep-side layer.
 38. The device according to claim 37, wherein each of theband gap energy of the n-side layer and a band gap energy of the p-sidelayer is respectively greater than a band gap energy of thelight-emitting layer.
 39. The device according to claim 37, wherein adifference between the band gap energy of the n-side layer and a bandgap energy of the active layer is not less than one third of adifference between a band gap energy of the p-side layer and the bandgap energy of the active layer.
 40. The device according to claim 37,wherein the n-side layer includes In, the p-side layer includes Al, andthe light-emitting layer includes at least one InGaN layer having athickness not greater than 0.1 micrometers.
 41. The device according toclaim 37, wherein the device has a double hetero structure, the n-sidelayer includes an n-clad layer, and the p-side layer includes a p-cladlayer.
 42. A GaN type semiconductor light emitting device having adouble hetero structure, the device comprising: a substrate; andcompound semiconductor layers stacked on the substrate, the layerscomprising: at least one n-side layer including at least onehole-confining layer and at least one n-type contact layer; at least oneactive layer; and at least one p-side layer including at least oneelectron-confining layer and at least one p-type contact layer, whereinat least one band gap energy of the active layer is smaller than atleast one band gap energy of each of the hole-confining layer and theelectron confining layer respectively, and the at least one band gapenergy of the hole-confining layer is smaller than the at least one bandgap energy of the electron-confining layer.
 43. The GaN basedsemiconductor light emitting device according to claim 42, wherein theactive layer includes at least one InGaN layer and a thickness of theInGaN layer is not greater than 0.1 micrometers.
 44. The GaN basedsemiconductor light emitting device according to claim 42, wherein eachof the n-side layer and the p-side layer includes at least one of Al andIn.
 45. The GaN based semiconductor light emitting device according toclaim 44, wherein the n-side layer includes In and the p-side layerincludes Al and In.
 46. The GaN based semiconductor light emittingdevice according to claim 42, wherein a difference between the at leastone band gap energy of the hole-confining layer and the at least oneband gap energy of the active layer is not less than one third of adifference between the at least one band gap energy of theelectron-confining layer and the at least one band energy of the activelayer.
 47. A GaN based semiconductor light emitting device comprising:means for injecting an electron into an active layer at low voltage;means for injecting a hole into the active layer; means for preventingleakage of the hole from the active layer; and means for recombining theelectron and hole to emit light.
 48. The GaN based semiconductor lightemitting device according to claim 47, wherein the means for injectingan electron includes an n-type material having a first band gap energy,the means for injecting a hole includes a p-type material having asecond band gap energy greater than first band gap energy, the means forpreventing leakage includes the active layer having a third band gapenergy less than the first band gap energy, the hole has a greatereffective mass than that of the electron.
 49. The GaN basedsemiconductor light emitting device according to claim 47, wherein thelow voltage is lower than an operating voltage of a prior artsemiconductor light emitting device having an n-clad layer ofAl_(0.5)Ga_(0.85)N.
 50. A method for emitting light comprising:injecting an electron into a gallium nitride type compound layer at alow voltage; injecting a hole into the layer; preventing leakage of thehole and the electron from an active layer; and recombining the hole andthe electron in the layer to emit light.
 51. The method according toclaim 50, wherein injecting an electron includes applying a voltage tocause an electron to flow through an n-type material having a first bandgap energy, injecting a hold includes applying a voltage to cause a holeto flow through a p-type material having a second band gap energygreater than the first band gap energy, preventing leakage includesconfining the hole in the gallium nitride type compound layer having athird band gap energy less than the first band gap energy, an effectivemass of the hole is greater than an effective mass of the electron andthe low voltage comprises an operating voltage lower than the voltagewould if, for the same second band gap energy, the first and second bandgap energies were equal.